Historically, modification of adhesive properties has been accomplished by altering adhesive parameters such as the type of formula components, ratios of components, coating thicknesses, cure schedules, internal reinforcements, etc. Although satisfactory in many respects, it is not always feasible to modify the adhesive or application of the adhesive. Accordingly, it would be desirable to provide a method of improving adhesive performance without modifying the adhesive formulation or parameters directly associated with the adhesive itself or its application.
Properties of particular importance concerning adhesives and their application are peel strength and shear performance. Generally, these properties can be modified to a certain extent. However, modification of these properties beyond a certain point adversely affects other properties of an adhesive. Specifically, it is often desirable to improve peel and shear strength at elevated temperatures. Although techniques for improving these characteristics are known, these techniques generally require the use of reinforcing resins or achieving particular degrees of crosslinking, one or both of which can detract from tack and general adhesion of the resulting adhesive. Accordingly, it would be beneficial to provide methods for improving peel strength and/or shear performance without significantly detracting from other properties of the adhesive.
Low density polyethylene (LDPE) film is particularly suited for use as a water shield or water deflector such as in automotive door assemblies due to the material's toughness, water resistance, and favorable strength and flexibility properties at both low and high temperatures. LDPE films have been used with pressure sensitive adhesives and have provided a cost effective solution to water intrusion for many vehicle component parts. Such films are often adhesively bonded to an underlying substrate. A significant problem related to the use of LDPE with certain pressure sensitive adhesives however, relates to heat characteristics. When the coefficient of thermal expansion of polyethylene does not match that of the substrate to which it is attached, such as many metals, the greater expansion upon heating of polyethylene can result in film buckling and debonding of the adhesive. For example, upon heating, a metal automotive door expands significantly less than polyethylene. A polyethylene part adhered to the door which may be 30 inches long at room temperature can expand to 31 inches long at 160° F. The excess expansion of the polyethylene imparts significant strain on the adhesive's ability to maintain adequate bond strength. Failure of the adhesive bond can result in the intrusion of water, dust and noise through the automotive component or door. Accordingly, it would be desirable to provide a technique for reducing the coefficient of thermal expansion of polyethylene so that polyethylene films could be used in conjunction with metal automotive components and conventional adhesives could maintain a bond between those materials.